CLIMATIC VARIETIES 165 



torial system, admittedly in a somewhat lax fashion, the genetic 

 interrelations of the types can be represented; but how it comes 

 about that each type maintains a high degree of integrity in 

 its own region we can only imagine. Each has in actual fact 

 a stability which the intermediate forms have not, but we cannot 

 yet analyse the nature of that stability. Mendelian conceptions 

 show us how by segregation the integrity of the factors can be in 

 some degree maintained, but not why certain combinations of 

 factors should be exceptionally stable. All that is left us to 

 fall back on is the old unsatisfying suggestions that some com- 

 binations may have greater viability than others, that there 

 may be a tendency for like to mate with like, and so forth. 



These difficulties acquire more than ordinary force in those 

 cases in which the two fixed types inhabit regions differing in 

 some respect so obvious and definite that we are compelled to 

 regard each type as climatic and as specially adapted to the 

 conditions. When for example an animal has a distinct type 

 never met with except in Arctic or Alpine conditions, and another 

 type proper to the plains and temperate regions, what are the 

 characteristics of the population of intermediate latitudes or 

 at intermediate levels? Some of the examples discussed in the 

 last chapter may be instances of this very nature, but even if 

 they are not, others are forthcoming which certainly are. The 

 evidence of these cases leads to the suspicion that with further 

 knowledge they will be found to consist of two classes, some in 

 which the observer as he passes from the one climate to the other 

 will find the intermediate area actually occupied by a population 

 of intermediate character, and others in which, though we may 

 presume the maintenance of intermediate conditions in the tran- 

 sitional area, there is no definite transitional population. This 

 interrupted or discontinuous distribution seems, so far as I 

 have means of judging, to be by far the more common of the two. 

 I do not doubt that by sufficient search individuals representing 

 every or almost every transitional form can be found, but it is 

 apparently rare that populations corresponding to these several 

 grades can be seen. The question has in few if any cases been 

 studied with precision sufficient to provide a positive answer; 



